This invention relates to a method for checking a spot welded portion for examining the quality of joint and a spot welding control system for controlling spot welding conditions by using this method.
The invention relates also to a spot welding machine for welding plates to be joined and, more particularly, to a spot welding machine capable of measuring thickness of plates between electrodes during spot welding.
Spot welding is suitable for welding thin metal plates efficiently and in a large quantity and is frequently employed in a production line system for products such as automobiles. In spot welding, however, it is not so easy to examine the quality of joint from appearance of a welded product as in gas welding or arc welding and there is no established method for checking the quality of joint without breaking a welded product.
Various studies and developments have therefore been made to realize a method for checking the quality of joint without breaking a welded product. One of these methods attempts to examining the quality of joint in a welded portion by measuring distance between welding electrodes (i.e., thickness of plates to be joined).
This prior art method will be described with reference to FIG. 10.
FIG. 10 shows a diagram showing states of change in the distance between electrodes in respective steps of spot welding.
Spot welding is performed in four steps initial pressing time (squeeze, time); power supply time, i.e. current supply time; hold time; and off time. The initial pressing time is a period of time between start of a application of pressing force to the electrodes and start of supply of current. Pressure between the electrodes is stabilized and plates to be joined are caused to conform to each other during this period of time. The power supply time, i.e. current supply time, is composed of a time period during which welding current is supplied, a time period during which preheating current is supplied and a time period during which tempering current is supplied to perform quenching after supply of welding current. The holding time is a period of time between the end of supply of current and a time point when the electrodes are disengaged from the welded plates. The off time is an interval between two spot welding processes when spot welding is repeatedly performed.
As will be apparent from the figure, the distance between the electrodes increases gradually after supply of current is started and is saturated at a maximum amount of change Hmax. During the hold time after the end of supply of current, the welded portion is cooled by the electrodes and the distance between the electrodes decreases sharply to a value which is proximate to the distance between the electrodes during the initial pressing time or even below this distance. When supply of current is continued after the distance between the electrodes has reached the maximum amount of change Hmax, fusing of the plates to be joined progresses and the distance between the electrodes starts to decrease gradually due to pressure between the electrodes but does not decrease sharply as when supply of current has ended as shown in the figure. Such change characteristic of the distance between the electrodes is an ideal one and an actual change characteristic varies in each spot welding with a resulting variation in the quality of joint in the welded plates.
As a method for examining the quality of control in spot welding by the maximum amount of change Hmax in the distance between the electrodes during supply of current obtained from this change characteristic curve, there is a method disclosed in Japanese Patent Publication No. 48-41422.
As a method for examining the quality of a joint by a rate of change (dh/dt) in the distance between the electrodes during an initial period of time in current supply, there is a method disclosed in U.S. Pat. No. 3,400,242.
As a method for examining the quality of a joint on the basis of both the maximum amount of change Hmax and the rate of change (dh/dt), there is a method disclosed in Japanese Patent Publication No. 53-4057.
An examination of an actual quality of a joint in a spot welding by using the prior art checking method has revealed that there is no clear correlation between the quality of joint and the maximum amount of change Hmax in the distance between electrodes during supply of current or the change rate (dh/dt) during an initial period of supply of current. In other words, there is a case where "poor quality of joint" results from the checking made on the basis of the maximum distance Hmax or change rate (dh/dt) notwithstanding that the quality of joint and welding strength which have been actually obtained are adequate or, conversely, a case where a "good quality of joint" results from such checking notwithstanding that the quality of joint and welding strength which have been actually obtained are poor.
Reliability of such checking method of a spot welding therefore is not sufficiently high and it is difficult to control welding conditions on the basis of results obtained by such checking method.
In a case where alloyed molten galvanized steel plates with a thickness of 0.65 mm are laid one upon the other and spot welded by using tip electrodes having a diameter of 5 mm under welding conditions of pressure of 170 kgf to 250 kgf and welding current 8600 A to 9800 A, the maximum amount of change Hmax is about 0.30 mm to 0.120 mm (30 .mu.m to 120 .mu.m). In order to realize the above described checking method of spot welding, therefore, the distance between the electrodes must be detected at an order of about 1 .mu.m during spot welding. It is extremely difficult to detect, at such a high accuracy as 1 .mu.m, the distance between electrodes through which current of several thousand amperes flows.
In the past, it has been experimentally performed to mount a linear sensor such as a potentiometer on an electrode holding member for a movable electrode of a spot welding machine and obtain relationship between displacement of the movable electrode (i.e., distance between the electrodes) and the quality of a joint in the spot welding by measuring the displacement of the movable electrode during spot welding by means of the linear sensor. It has however not been in practice to provide such linear sensor on a spot welding machine located on a production line. This is because the provision of a linear sensor on an electrode holding member causes hindrance to a welding work. Accordingly, there has not been a spot welding machine capable of performing a welding work while measuring change in the distance between electrodes and thereby checking the quality of a joint by spot welding because of lack in accuracy in the measurement of the distance between electrodes and difficulty in arranging a detector despite the fat that the above described various methods for checking spot welding have been proposed.
Further, in the past, a linear sensor such as a potentiometer has been experimentally mounted on an electrode holding member for a movable electrode of a spot welding machine and displacement of the movable electrode during welding has been measured by the linear sensor. This manner of mounting of a linear sensor is effective in a spot welding machine in which rigidity of an electrode holding member for a stationary electrode is so strong that the electrode holding means is not readily deformed by pressure applied thereto during welding. In a spot welding machine such as a portable type spot welding machine in which rigidity of arms which hold electrodes is not so strong or in a spot welding machine such as an X-shaped welding gun in which electrodes on both sides are movable, however, this manner of mounting of a linear sensor poses the problem that the distance between the electrodes cannot be detected accurately due to deformation of the arms.